t3 are currently AWS' bread and butter, general-purpose instances: they are fine for JATOS.
> Could you give an example of a demanding experiment? Do such experiments need more vCPUs?
Demanding experiments put a lot of load on the server and there are many ways to do this. This is just some common ways to do this:
* use the jatos.submitResultData or jatos.appendResultData a lot (like every couple seconds) - some experiment builders like OpenSesame/OSWeb often do this
* even worse: having big result data (in MB) in combination with many jatos.submitResultData or jatos.appendResultData
* have many experiments running at the same time, I mean literally having many participants (dozens or hundreds) doing studies in the same moment. E.g. if you post a study link on Twitter and have a thousand participant trying your study at the same time your server might get overloaded.
* all experiments that load or upload media files (video, audio, large images)
And two more things: You mentioned that you want to install MySQL on the same instance. No problems with that, but I'd suggest to